To create lighter hydrocarbons with greater commercial value, portions of crude oil are generally thermally or catalytically "cracked". Cracking produces light gaseous byproducts such as butylene, isobutane, propylene and propane. Other refining processes also produce some of these gaseous byproducts. These light gases are usually further processed to create larger hydrocarbon molecules of greater commercial value. One such process involves reacting isoparaffins with olefins to create "alkylates". For example, the alkylate produced by reacting an isoparaffin (e.g. an isobutane) with an olefin (e.g. isobutylene) consists primarily of 2-2-4 trimethylpentane, which is the molecule whose combustion defines an octane number of 100. Alkylates are generally highly branched hydrocarbons that have high octane numbers and desirable volatility characteristics. They are particularly useful as components of a reformulated gasoline ("RFG"). RFGs are required to meet the highly restrictive emission standards under the Clean Air Act.
Alkylates are commercially produced in oil refineries by reacting isoparaffins with olefins in the presence of anhydrous hydrofluoric acid or sulfuric acid, which acids serve as catalysts for the reaction. Boron trifluoride and other halides have been examined for use as catalysts to produce alkylate, but have not been commercially accepted. Hydrofluoric acid and sulfuric acid, however, have severe environmental shortcomings, which have led to ongoing efforts to find alternative processes for producing alkylates.
Solid catalysts have long been employed in the petroleum industry. They have, however, been ineffective in catalyzing the formation of isoparaffin alkylates because of the chemical inactivity of isoparaffins and because the solid catalysts rapidly accumulate "coke" deposits.
In other processes, such as the alkylation of benzene to produce ethyl benzene or cumene, the buildup of coke is relatively slow and solid catalysts such as ZSM-5 are satisfactory. This is because benzene and other aromatics are sufficiently reactive to be easily alkylated.
To the contrary, isoparaffins including isobutane are relatively inert or unreactive and have required special measures such as the use of strong acids to achieve alkylation. As indicated above, the use of strong acids such as hydrofluoric acid and sulfuric acid presents environmental problems.
It would therefore be a significant advance in the art of producing alkylates if the reaction of isoparaffins and olefins could be carried out using environmentally acceptable materials.